This invention relates to breathing apparatus, particularly for aircraft passengers.
In some aircraft accidents lives are lost due to the consequences of fire and in particular to the inhalation of smoke and fumes or hot gases. In some accidents passengers are overcome while still seated but do not show evidence of severe external burns. Some of these passengers could perhaps have been saved if a portable respirator, so called gas mask, had been available for each. Such devices can remove smoke and noxious gases by filtration and absorption. However, inhalation of very hot air, in the absence of noxious gases, can still cause death from thermal damage to the internal lining of the lungs. The use of a conventional gas mask would afford little or no protection against such very hot air. Furthermore in an aircraft fire noxious fumes from fuel and other sources may be present in such large amounts that normal filtration or absorption facilities can become blocked or exhausted. Additionally the fierce combustion of aircraft fuel can substantially reduce the oxygen content of the cabin atmosphere. In patho-physiological terms alveolar burns, pulmonary oedema and shock may contribute to acute anoxia which may be of anoxic, anaemic, stagnant or histotoxic type or to a combination of some or all of these. Lives might also have been saved if each passenger had a portable respirator with self contained compressed air cylinder, as used by firemen and subaqua divers, but such apparatus is not practicable because of cost, weight and a high degree of skill and training required to use it.
There are three main types of self contained breathing apparatus for aircraft passengers: firstly those which have filters to remove smoke and noxious gases, but these do not protect against hot gases; secondly those which provide a continuous supply of air or oxygen from a pressurised cylinder, but large volumes of gas are required and cylinders are heavy; and thirdly those which use pure oxygen in a rebreathing system with carbon dioxide absorber, but measures must be taken to maintain the absorbent material in good condition until it is needed and to remove nitrogen from the system.
The efficiency of the absorbent material is dependent upon many chemical, physical and other factors. Importantly, the material deteriorates and becomes exhausted if it is exposed to air but frequent replacement is very expensive in maintenance costs. The activity is preserved for many months if the absorbent is sealed to prevent contact with air, which contains carbon dioxide and water vapour, but this is not easy to do. There is a substantial amount of space between absorbent granules and this space is filled with air. If both ends of the container are sealed, the pressure within varies as the aircraft ascends and descends. This fluctuation may rupture the seal, even when there is a minimal volume of retained air.
It is dangerous to rebreath air in the presence of an absorber, for acute anoxia may occur without warning, but pure oxygen is safe. Nitrogen should therefore be reduced but this should be achieved without waste of oxygen. The apparatus will be used by passengers without training or experience and with very little instruction. Thus the system should be simple to use and where possible automatic. It may also have the oxygen source fixed to the conveyance means and may deliver oxygen to the reservoir. To reduce the risk of pure oxygen exacerbating a fire, an oxygen-rich atmosphere may be used rather than a pure oxygen atmosphere. If nitrogen wash out is not complete, the system is safe to use when the oxygen inflow is equal to or in excess of oxygen usage. If oxygen usage is in excess, then time is limited by the amount of oxygen remaining.
A harness with adjustable straps may be used to retain a face mask in position and obtain a good fit to prevent gas leaks. Provision must also be made for children and for infants.